The 14-page opinion has been posted here. For those who don’t have time to read it, here is a distillation of the arguments in the legal analysis section (Part II) of the opinion, in order of their appearance:

2.In light of the “gloss” of historical practice, the “vast share of responsibility for the conduct of our foreign relations” belongs to the President, who may act in that context without prior congressional authorization. More specifically, there is a substantial record of military deployments unsupported by prior congressional approval, including recent examples such as the bombing of Libya in 1986, the conflict in Panama in 1989, the deployment into Somalia in 1992, the deployment into Bosnia in 1995, deployments to Haiti in 1994 and 2004, air patrols and strikes in Bosnia from 1993 to 1995, and the bombing of Serbia in 1999. The functional need for quick decisions in this realm reinforces this conclusion.

3. The War Powers Resolution, with its 60-90 day timetable for the withdrawal of troops from hostilities in the absence of affirmative congressional approval, implicitly recognizes the power of the President to act unilaterally at least within that timeframe.

4. This unilateral authority does not necessarily extend to circumstances qualifying as “war” within the meaning of the Declare War Clause. But not all uses of military force come within the meaning of “war” as used in this context. What counts as war depends on “a fact-specific assessment of the ‘anticipated nature, scope, and duration’ of the planned military operations” (p. 8, quoting the 1994 OLC Haiti letter). In practical terms, this requires “prolonged and substantial military engagements, typically involving exposure of U.S. military personnel to significant risk over a substantial period.” (p.8).

We are not suggesting, however, that the United States cannot be said to engage in "war" whenever it deploys troops into a country at the invitation of that country’s legitimate government. Rather, we believe that "war" does not exist where United States troops are deployed at the invitation of a fully legitimate government in circumstances in which the nature, scope, and duration of the deployment are such that the use of force involved does not rise to the level of "war."

In deciding whether prior Congressional authorization for the Haitian deployment was constitutionally necessary, the President was entitled to take into account the anticipated nature, scope and duration of the planned deployment, and in particular the limited antecedent risk that United States forces would encounter significant armed resistance or suffer or inflict substantial casualties as a result of the deployment.(10) Indeed, it was the President’s hope, since vindicated by the event, that the Haitian military leadership would agree to step down before exchanges of fire occurred. Moreover, while it would not be appropriate here to discuss operational details, other aspects of the planned deployment, including the fact that it would not involve extreme use of force, as for example preparatory bombardment, were also relevant to the judgment that it was not a "war."]

5. In the two years leading up to the deployment of 20,000 troops to Bosnia in 1995, the US participated in air operations to enforce a no-fly zone and to protect civilians—including an intense two week period involving attacks on “hundreds of targets” by NATO air assets (with US participation). OLC concluded at the time that the subsequent deployment of ground forces for peacekeeping presented a harder issue as to the existence of “war,” but still concluded that the threshold was not crossed.

6. The two key questions with respect to Libya today, therefore, are:

(i) Whether the U.S. has sufficient interests at stake to implicate the unilateral power of the President to use military force, and

(ii) Whether the nature of the force being used crosses the “war” threshold.

7. Sufficiency of US interests:

The “combination of at least two national interests” at stake here (as reasonably determined by the President) satisfy the first inquiry: “preserving regional stability and supporting the UNSC’s credibility and effectiveness.” (p. 10)

As to regional stability, this goes to both national security and foreign policy concerns (citing previous reliance on this rationale by OLC in relation to Kosovo, Bosnia, and Haiti). The memo cites several risks in this regard. First, it emphasizes the risk that failure to stop Ghaddafi might have produced large refugee flows that might destabilize neighboring states and possibly spark wider conflict. Second, the memo also notes the humanitarian threat to civilians had there been no intervention, though it is not clear whether OLC means to assert a stand-alone national interest in preventing atrocity as a sufficient condition for triggering this authority. Third, the memo notes that failure to stop Ghaddafi might have “encouraged the repression of other democratic uprisings.”

As to supporting UNSC credibility/effectiveness: the memo explains that the United States at least since Korea has taken the position that it is important to our national interest that the UNSC maintain its credibility.

8. Whether U.S. involvement in Libya amounts to “war””

U.S. involvement in Libya is too limited to be a “war” in terms of the Declare War clause. First, the involvement is limited to air power, and does not entail ground troops where withdrawal is difficult (making it harder for Congress to object post-deployment) and escalation easily foreseeable. Second, the goals are limited and do not include territorial occupation or conquest. Third, though of course one cannot say there is no risk of sustained military conflict, the bombing is not bombing in the nature of preparation for a ground invasion; the current situation is well within the precedents of Bosnia (17 days of bombing) and Serbia (two months of it).

9. Note that the memo does not address whether the Administration believes that it will have to stop operations upon expiration of the 60-90 day “clock” under the War Powers Resolution.